1. Field of the Invention
The present invention concerns liquid crystalline polymers and processes for the production thereof. The present invention also relates to blends of liquid crystalline polymers and thermoplastics.
2. Description of Related Art
All liquid crystalline (LC) polymer phases have in common that chain segments or mesogens adopt a more or less parallel alignment. LC main chain polymers primarily consist of para-substituted aromatic building blocks which favour linear conformations in combinations with suitable links such as ester, amide, azo or ethylene groups. Therefore, all known thermotropic main-chain LCP's comprise straight and stiff chains with only small structural deviations from “rigid rods” obtained by kinks, bends, etc. which are necessary for the melt processing.
The straight segments of the LC polymers, also called the “mesogens”, easily find each other and form the “mesophase” which is characteristic for all LCP:s. In a mesophase, the mesogens are oriented in the same direction which is the direction of the melt flow. The chains are therefore placed in parallel alignment beside each other and with essentially no inter-mingling or formation of entanglements. This means that the properties in flow direction and transverse direction, respectively, are very different. The phenomenon is called “anisotropy” and it constitutes one of the greatest disadvantages of LCP's and LCP-blends. Thus, for example, the tensile strength of an LCP is very good in the flow direction but extremely bad in the transverse direction. Due to the lack of entanglements the melt elasticity and the melt strength are also almost non-existent. As a result, an extended melt does not snap back into its original shape. This very detrimental effect on the melt flow also makes it almost impossible to obtain even thickness profiles in films, and in blown-film extrusion the bubble is very unstable. The problem with uneven thickness profiles becomes even worse when LCP's are coextruded with normal isotropic polymers. Further, it is almost impossible to apply many other melt processing techniques like blow-moulding, thermoforming etc. to LCP's and LCP-blends due to the lack of the necessary melt-strength.
It is known in the art that the degree of anisotropy can be slightly decreased by adding fillers and/or isotropic polymers (LCP-blends), but the results obtained are not satisfactory. Technical solutions for making isotropic LCP-products include the use of a rotating die, a cone-extruder and various other mechanical means which create different orientations of the LCP-chains at different depths in the product. In this way isotropic solid state properties can be obtained but still no melt elasticity and melt strength.
In liquid crystalline polymers, monomers having both functional groups in ortho- or meta-position are considered to be interrupting units which severely destabilize the LC phase. It is known in the art that oligomers can be built up from catechols and a dicarboxylic acid [Kricheldorf, H. R. et al. Macromolecules 26 (1993) 5161–5168]. The molecular weights of these known oligo(ester imide)s were low (DP<2000 Da) for unknown reasons. Based on the results obtained for liquid crystalline oligomers no distinct conclusions can be drawn for polymers of technically interesting molecular weights.
Technical polymers containing ortho- or meta-substituted compounds are also known in the art.
Thus, EP 0 380 286 (Nippon Oil) discloses the use of divalent aromatic ortho-compounds (meta compounds as references) in wholly aromatic polyester type LCP:s in order to lower the melting point without impairing the mechanical properties. Here these ortho-compounds (catechol, ortho-phthalic acid, salicylic acid etc.) distort the linearity of the LCP-chains so that the melting points are reduced to below their degradation temperature making them melt processable. However, in EP 0 380 286 the effect of the ortho-compounds on the melting point is rather small. The lowest melting point obtained (Example 2) was as high as 341° C. which makes these LCP:s totally unsuitable for any extrusion application.
EP 0 265 240 (Polyplastics) suggests using two or more naphthalene based monomers in order to improve the mechanical properties of LCP:s. By doing so it is especially the flexural strength and the coefficient of linear expansion which become more balanced in the machine direction and the transverse direction in injection moulding. WO 97/34964 (Hoechst Celanese) teaches the use of isophthalic acid in order to facilitate polycondenzation of polyesteramide type LCP:s containing hydroquinone as monomer. U.S. Pat. No. 5,454,910 (Korea Institute of Science and Technology) discloses a method for manufacturing LCP-fibres with certain characteristics. Here metahydroxybenzoic acid has been used.
JP 07292362 (Sumitomo Chemical) comprises blends of LCP and syndiotactic polystyrene.
JP 07233249 (Nippon Oil) comprises LCP:s for optical devices with controlled birefringence.
In summary, none of the above mentioned publications teaches or suggests a technical solution for providing (improved) melt strength in LCP's which would allow for extrusion of the polymers. The property of melt strength is crucial in all extrusion processes, but in particular in coextrusion and alloying with isotropic polymers.